This invention relates generally to a drive circuit for a brushless motor, and in particular, to a control circuit for stabilizing the operation of a brushless motor drive circuit. A motor construction incorporating hall effect elements to detect the position of the rotor is depicted in U.S. application Ser. No. 280,705, now U.S. Pat. No. 3,831,071 of Yoshihiru Mitsui, and is incorporated by reference herein.
While control circuits comprised of several transistors for controlling the speed and operating characteristics of a brushless motor drive circuit have been provided, such control circuits have heretofore been less than completely satisfactory. Because it is difficult to determine the exact operating point of each transistor and because such inability is multiplied by the use of more than one transistor, the operating range of the control circuit is extremely limited. Moreover, it is increasingly difficult to maintain the stability of the drive circuit over a wide range of operating voltages in view of the variations in the voltage supplied by the power source. Accordingly, it is desirable to produce a brushless motor drive circuit which includes a control circuit formed of a single transistor and is substantially unaffected by normal fluctuations in the supply voltage.